Because of thermal stability limitations, longitudinal recording is not expected to be suitable to achieve a 100 Gbit/in.sup.2 /recording density. Reduced demagnetizing fields in the recorded transition make perpendicular magnetic recording more favorable at such a high recording density.
Thus, extensive research on the subject of perpendicular magnetic recording has been ongoing for more than a decade. Although perpendicular recording was predicted in the late 1970s to be able to achieve higher recording density than longitudinal recording, current technology still favors magnetic easy axis in-plane oriented media.
To achieve the perpendicular mode at high recording densities, the recording medium is required to have not only a large perpendicular anisotropy, but also good chemical stability and high mechanical hardness. Barium hexaferrite is a very promising candidate to provide these characteristics.
Indeed, many research efforts have been made to deposit high quality barium hexaferrite films. See e.g., M. Matsuoka, Y. Hoshi, M. Naoe and S. Yamanaka, IEEE. Trans. Mag., Mag-18, 1119-1121 (1982); M. Matsuoka, Y. Hoshi, M. Naoe and S. Yamanaka, IEEE Trans. Mag., MAG-20, 800-802 (1984); A. Morisako, M. Matsumoto and M. Naoe, IEEE Trans. Mag., MAG-23, 56-58 (1987); M. Matsuoka and M. Naoe, J. Appl. Phys. 57 (1), 4040-4042 (1985); and M. Matsuoka and M. Naoe, IEEE Trans. Mag., MAG-21, 1474-1476 (1985). These studies have used target facing type sputtering (TFTS) and conventional rf or dc diode sputtering with in-situ annealing.
A current popular technique for producing barium hexaferrite films for perpendicular recording and magneto-optical recording is TFTS. Although this method produces good c-axis perpendicular orientation, there are several significant drawbacks. First, the sputtering machines required are not commonly used in industry, impeding commercialization of the process. Moreover, the TFTS method uses a very high in-situ temperature during deposition to crystallize the material (typically around 600.degree. C.), which is an additional impediment to commercialization. Still further, the complicated interfacial structure between barium hexaferrite and silicon dioxide, caused by interdiffusion of barium into silicon dioxide, makes it difficult to fabricate very thin barium hexaferrite films. E. Lacroix, P. Gerard, G. Marest and M. Dupuy, J. Appl. Phys., 69, 4770-4772 (1991).
For microwave/millimeter wave applications, a number of researchers have attempted to grow barium hexaferrite epitaxially on single crystal sapphire substrates. The epitaxial film is then used as a seed layer for the growth of a much thicker film using the liquid phase epitaxy (LPE) technique. M. S. Yuan, H. L. Glass and L. R. Adkins, Appl. Phys. Lett., 53(4), 340-341 (1988). However, it has been reported that the seed layer gives a non-uniaxial-like torque curve in the perpendicular torque magnetometer measurement, indicating imperfect c-axis perpendicular orientation. T. L. Hylton, M. A. Parker, K. R. Coffey and J. K. Howard, paper EC-06, submitted to Magnetism and Magnetic Materials Conference, Houston, Tex. (Dec., 1992). It has also been reported that aluminum starts to diffuse from sapphire into barium hexaferrite above 1000.degree. C., which makes LPE growth of pure barium hexaferrite by this method questionable. T. L. Hylton, M. A. Parker and J. K. Howard, Appl. Phys. Lett., 61, No. 7,867-869 (1992). Typical LPE processes normally involve very high temperatures (above 1000.degree. C.).
Thus, although barium hexaferrite thin films are attractive materials for potential applications in magnetic recording, magneto-optical recording and microwave/millimeter wave devices, several materials processing problems must be resolved to develop a method for controlling the crystal orientation of barium hexaferrite thin films in a manner amenable to commercial production.